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Isotope studies of granitoids from the Bangenhuk Formation, Ny Friesland Caledonides, Svalbard

Published online by Cambridge University Press:  01 May 2009

Å. Johansson
Affiliation:
Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50 007, S-104 05 Stockholm, Sweden
D. G. Gee
Affiliation:
Department of Geophysics, Uppsala University, Box 556, S-751 22, Uppsala, Sweden
L. Björklund
Affiliation:
Department of Geology, University of Göteborg, S-412 96 Göteborg, Sweden
P. Witt-Nilsson
Affiliation:
Department of Geology, University of Lund, Sölvegatan 13, S-223 62 Lund, Sweden

Abstract

The Caledonian Hecla Hoek succession in Ny Friesland, eastern Svalbard has been interpreted, for many decades, to be a continuous stratigraphic sequence. Early Palaeozoic and Neoproterozoic strata in its upper parts pass more or less conformably down into amphibolite facies rocks (Stubendorffbreen Supergroup) at depth. Recent isotopic age-determination and structural studies have indicated that the Stubendorffbreen succession is tectonostratigraphic and made up of at least three major thrust sheets. This paper provides new data from two meta-igneous units within the succession, the Bangenhuk and Instrumentberget gneisses. Both are granitoid sheets, consisting mainly of red, strongly lineated gneisses of monzogranitic composition; the Bangenhuk unit also contains some lenses of little deformed granitoids, as well as cross-cutting aplite dykes, amphibolitized dolerites and subordinate metasedimentary rocks. The latter are locally cut by granitoids. U—Pb zircon dating of six samples of variably deformed Bangenhuk granitoids, including one cross-cutting aplitic dyke, has yielded ages between 1720 and 1770 Ma, the higher values generally from the less deformed samples. The Instrumentberget gneissic granite yielded an age of 1737+46−41 Ma. These ages are interpreted to date the time of intrusion of the granitoids at around 1750 Ma; the younger ages may have been slightly lowered by Caledonian deformation, particularly those from specimens located close to a major fracture (the Billefjorden Fault Zone) in Wijdefjorden—Austfjorden. U—Pb dating of titanite from the least deformed granitoid also yields comparable Palaeoproterozoic ages; in the more deformed rocks, however, titanites give evidence of Caledonian ductile deformation at c. 410 Ma. The Rb—Sr system of the corresponding whole rock samples has been disturbed and yields an errorchron age of about 1650 Ma and, for some samples, an impossibly low initial Sr ratio. The Sm—Nd system may be more intact and yields initial εNd values of −2 to −3, suggesting some contribution from older crustal material to the granitoid magmas. The results indicate the presence of extensive units of Palaeoproterozoic granitic basement within the Lower Hecla Hoek succession of Ny Friesland, supporting the hypothesis that the latter is composed of tectonically intercalated basement and cover units.

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Articles
Copyright
Copyright © Cambridge University Press 1995

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